This invention relates to electrolytic production of aluminum and more particularly, it relates to an improved cathode suited for use in an electrolytic cell for the production of aluminum such as Hall-Heroult electrolytic cells.
In the electrolytic production of aluminum, there is great interest in utilizing a cathode that is highly conductive and does not react with molten aluminum deposited thereon. Carbon cathodes which are traditionally used in the Hall-Heroult cells have the problem that they are not readily wettable with molten aluminum. The carbon cathode surface reacts with the molten aluminum and forms aluminum carbide. Thus, the cathode is depleted at about 2 to 5 cm/yr. during operation of the cell. Or, the carbon cathode has the problem that it permits formation of cyanides, presenting a disposal problem. Thus, the carbon cathode has been replaced or modified with materials to improve its performance.
For example, U.S. Pat. No. 5,961,811 discloses an improved carbonaceous material suitable for use as a cathode in an aluminum producing electrolytic cell, the cell using an electrolyte comprised of sodium containing compounds. The carbonaceous material is comprised of carbon and a reactive compound capable of suppressing the formation or accumulation of sodium cyanide during operation of the cell, of reacting with sodium to reduce problems associated with sodium intercalation, and of reacting with one of titanium or zirconium to form titanium or zirconium diboride during operation of the cell to produce aluminum.
U.S. Pat. No. 5,217,583 discloses electrodes suitable for electrochemical processing which are a preferred product form, particularly electrodes for use in the electrowinning of aluminum from its oxide. According to the patent, such products are comprised of a dimensionally stable combustion synthesis product of a composition containing at least 20% by weight of a particulate combustible material; at least 15% by weight of a particulate filler material capable of providing desired mechanical and electrical properties; and up to 35% by weight of a particulate inorganic binder having a melting point lower than the combustion synthesis temperature.
U.S. Pat. No. 4,243,502 discloses a wettable cathode for an electrolytic cell for the electrolysis of a molten charge, in particular for the production of aluminum, where the said cathode comprises individual, exchangeable elements each with a component part for the supply of electrical power. The elements are connected electrically, via a supporting element, by molten metal which has separated out in the process. The interpolar distance between the anodes and the vertically movable cathode elements is at most 2 cm.
U.S. Pat. No. 4,376,029 discloses a cathode component for a Hall aluminum cell which is economically produced from a mixture of a carbon source, preferably calcined petroleum coke, and optionally calcined acicular needle petroleum coke, calcined anthracite coal; a binder such as pitch including the various petroleum and coal tar pitches; titanium dioxide, TiO2; and boric acid, B2O3 or boron carbide, B4C; forming said mixture into shapes and heating to a TiB2-forming temperature.
U.S. Pat. No. 4,439,382 discloses that titanium diboride graphite composite articles are produced by mixing TiO2, petroleum coke and a binder to form a plastic dispersion. Articles are shaped by molding or extrusion and baked to carbonize the binder to form a baked carbon-TiO2 composite. The article is impregnated with a molten or dispersed boron compound, then heated to drive TiB2 forming reaction. The article is then further heated to a graphitizing temperature to form a graphite-TiB2 composite useful as a cathode component in a Hall aluminum reduction cell.
U.S. Pat. No. 4,456,519 discloses an electrode made of a number of elongated elements which are plates, rods or tubes. The elements are composed of inorganic conductive fibers embedded in a solid, electrochemically active material. The fibers are oriented in the direction of current flow.
U.S. Pat. No. 4,465,581 discloses that TiB2-graphite composite articles suitable for use as cathode components in a Hall aluminum reduction cell are made by impregnating a TiO2-carbon composite with a boron compound and carbon black dispersed in water, or alternately by impregnating a boron or boron compound-carbon composite with a carbon black-TiO2 dispersion, and heating the article to a reaction temperature whereby TiB2 is formed and the amorphous carbon converted to graphite. The article may be impregnated with a carbonizable liquid, re-baked, and re-heated to a graphitizing temperature to increase its strength and density.
U.S. Pat. No. 4,478,693 discloses an inert type electrode composition suitable for use in the electrolytic production of metals such as aluminum. The aluminum is produced from an aluminum-containing material dissolved in a molten salt. The electrode composition is fabricated from at least two metals or metal compounds combined to provide a combination metal compound containing at least one of the group consisting of oxide, fluoride, nitride, sulfide, carbide or boride.
U.S. Pat. No. 5,129,998 discloses that the density of various refractory hard metal articles are controlled so that articles made from the refractory hard metals are able to float on the surface of molten aluminum. Floating such articles on aluminum has been found to both stabilize and protect the surface of molten aluminum. Floating cathodes for use in aluminum reduction cells is a particular application for the floating refractory hard metals.
U.S. Pat. No. 5,527,442 discloses a carbonaceous, refractory or metal alloy substrate material coated with a refractory material, the refractory material including at least one of borides, silicides, nitrides, aluminides, carbides, phosphides, oxides, metal alloys, inter-metallic compounds and mixtures of one of titanium, chromium, zirconium, hafnium, vanadium, silicon, niobium, tantalum, nickel, molybdenum and iron and at least one refractory oxide of rare earth metals. An aluminum production cell including a component made up of a material coated with the coating described above is also disclosed.
U.S. Pat. No. 5,538,604 discloses an improved carbonaceous material suitable for use as a liner in an aluminum producing electrolytic cell, the cell using an electrolyte comprised of sodium containing compounds and the carbonaceous material penetrable by sodium or nitrogen and resistant to formation or accumulation of sodium cyanide during operation of the cell. The carbonaceous material is comprised of carbon and a reactive compound capable of reacting with one of sodium, nitrogen and sodium cyanide during operation of the cell to produce aluminum, the reactive compound present in an amount sufficient to suppress formation or accumulation of cyanide compounds in the liner.
U.S. Pat. No. 5,006,209 discloses that cathodes for use in low temperature cells 660xc2x0 to 800xc2x0 C. are typically composed of an electrically conductive, refractory hard metal which is wet by molten aluminum and stands up well in the bath under operating conditions and that the preferred cathode material is titanium diboride. U.S. Pat. No. 4,865,701 discloses that other useful cathode materials include titanium carbide, zirconium carbide, zirconium diboride, niobium diboride, tantalum diboride and combinations of said diboride in solid solution form, e.g., (Nb, Ta)B2.
In spite of these disclosures, there is still need for an improved cathode suitable for use in an electrolytic cell for producing aluminum.
It is an object of the invention to provide an improved cathode for use in an electrolytic cell for reducing alumina to aluminum in a molten salt.
It is another object of the invention to provide a cathode comprised of a base material having high electrical resistivity for use in an electrolytic cell for reducing alumina to aluminum in a molten salt.
It is yet another object of the invention to provide a composite cathode comprised of a base material having high electrical resistivity and having a reaction layer thereon.
Still, it is another object of the invention to provide a composite cathode comprised of a boron carbide or zirconium oxide base material and a layer wettable with molten aluminum.
Yet another object of the invention is to provide a cathode comprised of a base material having high electrical resistivity for use at a temperature above 900xc2x0 C. in an electrolytic cell for producing aluminum from alumina dissolved in a molten salt.
Still it is another object of the invention to provide a cathode comprised of a base material having high electrical resistivity suitable for reaction with molten aluminum to provide an aluminum wettable layer for use in an electrolytic cell for reducing alumina to aluminum in a molten salt.
These and other objects will become apparent from a reading of the specification and claims and an inspection of the drawings appended hereto.
In accordance with these objects, there is provided a method of producing aluminum from alumina in an electrolytic cell comprising the steps of providing a molten salt electrolyte in an electrolytic cell having alumina dissolved therein, the molten electrolyte having a surface and having a frozen crust thereon, the cell having a bottom and sides extending upwardly from the bottom to contain the electrolyte. The method includes providing an anode extending through the surface into the electrolyte. A cathode is provided on the bottom of the cell and the cathode is comprised of a base material having low electrical conductivity or high electrical resistivity. The base material is reactive with molten aluminum to form a reaction layer wettable with aluminum. Thus, in operation a layer of molten aluminum is provided thereon. Means such as a cathode bar extends from the layer of molten aluminum to bus bar outside the cell to conduct electrical current from the cell. The cathode bar can extend from the layer of molten aluminum through the reaction layer through the base material outside the cell to conduct electrical current from the cell. In the method, electrical current is passed from the anode through the electrolyte to the cathode, thereby reducing alumina in the electrolyte and depositing aluminum at the cathode.
The electrolyte preferably is molten at a temperature over 900xc2x0 C. When the base material is boron carbide, for example, molten aluminum is reactive therewith to form a layer containing aluminum boride wettable with molten aluminum. The anode may be comprised of carbon or cermet or other material which can function as an anode. The cathode can be prepared by providing a base material having low electrical conductivity or high electrical resistivity such as boron carbide and contacting or reacting the surface of the base material to provide a layer such as aluminum boride wettable with molten aluminum. This permits low electrically conductive material having high stability in molten aluminum to function as a cathode.